MYSTERIOUS emissions of gamma rays at the heart of our galaxy could be a clue that some of the universe鈥檚 dark matter takes the form of exotic stars made of invisible 鈥渕irror matter鈥�.
The existence of mirror matter is hotly debated. It was originally invoked to restore nature鈥檚 flawed left-right symmetry which, for instance, results in a neutrino with left-handed spin but not a right-handed one. For every known particle there would be a mirror partner of a similar mass 鈥� for example, a mirror electron for the electron. But evidence to back up this idea is sparse.
The gamma rays that might strengthen the case were picked up by the European Space Agency鈥檚 Integral space telescope. The rays have an energy of 511 kiloelectronvolts, which strongly suggests that they are produced when an electron and its antimatter partner, a positron, collide and annihilate each other, leaving only the flash of energy. The mystery is how positrons could be generated fast enough for this to happen. Integral鈥檚 observations suggest a rate of about 1043 electron-positron annihilations per second.
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The most obvious source of positrons is ordinary supernovae, but even these exploding stars cannot provide enough positrons. Another, more exotic, explanation is that the electrons and positrons come from the decay of a particle of dark matter (New 杏吧原创, 4 October 2003, p 8). Now Robert Foot of the University of Melbourne, Australia, and Zurab Silagadze of the Budker Institute of Nuclear Physics in Novosibirsk, Russia, say gamma rays with an energy 511 keV can be better explained by the explosion of stars made of mirror matter ().
Because mirror matter particles do not interact with ordinary particles they are prime candidates for dark matter. Crucially, however, mirror photons do interact with ordinary particles, albeit very weakly. 鈥淭his is the key to our proposal,鈥� says Foot.
Inside a mirror supernova, the vast number of mirror photons produced would create a significant number of normal electron-positron pairs. Unaffected by the mirror nuclei around them, these would explode out through the star, eventually annihilating to create the 511 keV gamma rays observed by Integral. 鈥淭he annihilation radiation can be explained by just one mirror supernova every 10 to 100 million years,鈥� says Foot.
Foot and Silagadze make a prediction. They say that better observations will show the 511 keV emission is not uniform across the galaxy, but rather confined to localised sources.